""Just be aware that egg withdrawal is 2 weeks minimum with ivermectin, and eggs from treated birds may never be sold. Our vet reckoned you shouldn't ever eat their meat if you'd treated them with ivermectin.""
""Seems strange since it's used for humans as well
""
""Products are not necessarily quite the same even though they are sold under the same name when you use them on different animals. Spot On for cattle and sheep would not be good for your dog for example.""
As you correctly point out Harmony - Spot on for dogs is a completely different product to Spot on as used on farm animals.
Ivomectin, however, is the generic term for a specific wormer and there is no confusion as to which wormer it is. So Ivomec as prescribed for humans (and it's taken orally) is the same product that you can use on poultry. Now I don't see how a few drops applied to a hen can possibly cause any effect on a human by the time it's gone though the hen's body. This dosage is negligible compared to the safe human dosage! And I certainly don't see how these few drops make said hen unfit for human consumption for the rest of its life.
I believe people make these sweeping statements about withdrawal periods because there has been insufficient research done on Ivermectin in poultry and the relevant information on withdrawal periods (if any) isn't available.
I personally put a couple of drops of Ivermectin on the top of the head/neck of affected birds, and carry on eating the eggs as usual. After all, I wouldn't be worried about a few drops landing on my skin, so why should I worry about any microscopic amount getting into the eggs?